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San Dionisio, a municipality in the province of Iloilo, Philippines, occupies a coastal position on the island of Panay. Situated at 11.2711°N, 123.0948°E, the terrain is generally flat, characteristic of a lowland area bordering the Visayan Sea. The municipality’s elevation is low, contributing to limited natural dispersion of pollutants. The surrounding landscape is a mix of agricultural lands – primarily rice paddies and coconut plantations – and mangrove forests along the coastline. This agricultural dominance introduces potential sources of particulate matter from farming activities, including burning of agricultural waste. The urban-rural gradient is relatively sharp; San Dionisio’s poblacion represents a concentrated area of commercial and residential activity, while the surrounding areas remain largely rural. Proximity to the sea influences local weather patterns, bringing in sea breezes, but also potentially trapping pollutants during calm conditions. The regional position within the Western Visayas means it’s susceptible to influences from larger industrial centres in nearby provinces, though San Dionisio itself is not heavily industrialised. The coastal location also makes it vulnerable to saltwater intrusion, impacting vegetation and potentially contributing to aerosol formation. Understanding this interplay of geography and land use is crucial for assessing and mitigating air quality challenges.

San Dionisio experiences a distinct wet and dry season, heavily influenced by the southwest monsoon (habagat) and the northeast monsoon (amihan). Air quality generally improves during the wet season (roughly June to November) as rainfall effectively washes away particulate matter and suppresses dust. However, increased humidity can also promote the formation of mold and other biological pollutants. The dry season (December to May) typically sees a decline in air quality. This is due to increased dust from agricultural activities, drier conditions facilitating the spread of pollutants, and potential for bushfires. February to April are often the most challenging months, with prolonged sunshine and minimal rainfall. Temperature inversions, where a layer of warm air traps cooler air near the surface, can exacerbate pollution during calm mornings in the dry season. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during these periods. The amihan winds can sometimes bring in pollutants from other areas, while habagat winds generally disperse them. Monitoring wind direction is key. Avoiding outdoor exercise during peak heat hours and staying hydrated are recommended health precautions.